I had this act up again a week ago. Took a business partner out to lunch and it made it really loud the whole time, very embarrassing, then by the time I got off work and took it to the dealer it completely stopped! I was pissed! Had the guy there look at it with me for about 20mins and not a peep.
Not sure if it's rocks but I was looking in the wheel/wheel well today (thought I heard it but it was actually the super gusty winds making weird noises going around and under the car) and I did notice if you flick the black metal pan (for lack of better words) that is right behind the disc brake, it'll make the exact same noise. So the idea of something caught in between that and the brake disc does seem more valid to me now.
I can sort of reproduce the noise. maybe next time I just throw rocks in there myself so the dealer will take the wheel off and look!
